Disparate performance in diff methods

[arlk]

First of all, thank you for this great package! I'm a relatively new Julia user and I was using Static Arrays for some polynomial math that needs to use diff. I noticed that the benchmark speeds are very different for the two diff methods. MWE:

julia> A = @SMatrix randn(3,20);

julia> @btime diff($A,dims=$2);
  3.299 μs (2 allocations: 1.06 KiB)

julia> @btime diff($A,dims=$(Val(2)));
  37.283 ns (0 allocations: 0 bytes)

[andyferris]

Yes, this is known... but that's a larger discrepency then I expected.

When constant propagation works a bit better, we should be able to make the first method fast and deprecate the second.

[c42f]

Looking at the generated code it looks like our explicit @inline is being ignored for both these methods. Is that expected?

[c42f]

Probable dup of #540? (keyword functions not getting inlined?)